1. Field of the Invention
The present invention relates to a method of preparing a coalesced spandex. More particularly, it relates to a method for dry-spinning a solution of a high-melting thermoplastic polyurethane, bundling the resulting spandex, and fusing the resulting bundle to form the coalesced multifilament spandex by a method which does not employ false-twisting.
2. Description of the Background Art
In the conventional production of a coalesced spandex by dry spinning, a solution of polyurethane or polyurethaneurea is prepared and extruded through spinneret holes into a spinning column. Heat is applied to the inside of the column to drive off the solvent and form filaments. Such filaments are customarily quite small in diameter in order to permit rapid evaporation of solvent. In order to make filaments with larger diameters and to improve the uniformity of the final product, a plurality of filaments are normally bundled together and cohered to each other ("coalesced") along their lengths by passing them through a jet such as described in U.S. Pat. No. 3,353,344. Such a false twist coalescence method is described in, for example, U.S. Pat. No. 3,094,374. Due to the random distribution of any nonuniformities along the length of individual filaments, such irregularities are effectively cancelled out when a number of filaments are thus coalesced, and the resulting coalesced multifilament spandex has improved uniformity over the individual filaments.
European published Patent Application Number 756026 discloses a method wherein immediately after dry-spinning of segmented polyurethaneureas which are not thermoplastic, the filaments are slightly bonded by passing them through a "thread control element with comb-like shape" immediately after spinning to form a multifilament which can be readily and easily split into single filaments.
Japanese published Patent Application Number 53-139847 describes melt-spinning of a low-melting polyurethane from widely spaced spinneret holes and the bonding of the resulting filaments into a coalesced multifilament spandex by passing them through a guide placed at a selected distance from the face of the spinneret.
Spandex is widely used in various applications such as apparel because it has desirable characteristics including high stretch and recovery. Expanding applications have led to new needs such as high uniformity combined with mechanical properties that are available from, for example, thermoplastic polyurethanes and polyurethaneureas. Spandex with desirable properties such as good heat settability, high elastic recovery, and good resistance to environmental conditions can be be prepared by dry-spinning a high-melting thermoplastic polyurethane to make a spandex such as that disclosed in International Patent Application Number WO95123883. Good heat settability is an advantage, for example, when the spandex is to be used in combination with other fibers such as wool which should not be exposed to the temperatures necessary to heat-set such spandex.
However, spandex uniformity tends to be unsatisfactorily low when filaments of such a polyurethane are dry-spun and coalesced by the conventional false-twist coalescence method, perhaps due to fluctuations in the twisting force. As a result, the combination of good uniformity with the desirable properties of a dry-spun thermoplastic polyurethane is still needed.